Funding for this research was provided by:
Ministry of Education, Culture, Sports, Science and Technology (16H02643)
Text and Data Mining valid from 2019-04-08
Received: 20 November 2018
Accepted: 19 March 2019
First Online: 8 April 2019
Ethics approval and consent to participate
: The study was approved by the Regional Ethics Committee at The University of Tokyo, Japan. Informed consent was not required for secondary use of data from the National Fertility Survey.
: Not applicable.
: All authors have completed the Unified Competing Interest form at ExternalRef removed (available on request from the corresponding author) and declare no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years, no other relationships or activities that could appear to have influenced the submitted work.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.